Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft, either directly or through the use of a gearbox, to a generator. This way, the generator produces electricity which can be supplied to the electrical grid.
A variable speed wind turbine may typically be controlled by varying the generator torque and the pitch angle of the blades. As a result, aerodynamic torque, rotor speed and electrical power will vary.
In a first operational range, from the cut-in wind speed to a first wind speed (e.g. approximately 5 or 6 m/s), the rotor may be controlled to rotate at a substantially constant speed that is just high enough to be able to accurately control it. The cut-in wind speed may be e.g. approximately 3 m/s.
In a second operational range, from the first wind speed (e.g. approximately 5 or 6 m/s) to a second wind speed (e.g. approximately 8.5 m/s), the objective is generally to maximize power output while maintaining the pitch angle of the blades constant so as to capture maximum energy. In order to achieve this objective, the generator torque and rotor speed may be suitably varied.
In a third operational range, which starts at reaching nominal rotor rotational speed and extends until reaching nominal power, the rotor speed may be kept constant, and the generator torque may be varied to such effect. In terms of wind speeds, this third operational range extends substantially from the second wind speed to the nominal wind speed e.g. from approximately 8.5 m/s to approximately 11 m/s.
In a fourth operational range, which may extend from the nominal wind speed to the cut-out wind speed (for example from approximately 11 m/s to 25 m/s), the blades may be rotated (“pitched”) to maintain the aerodynamic torque delivered by the rotor substantially constant. In practice, the pitch may be actuated such as to maintain the rotor speed substantially constant. At the cut-out wind speed, the wind turbine's operation is interrupted.
In the first, second and third operational ranges, i.e. at wind speeds below the nominal wind speed (the sub-nominal zone of operation), the blades are normally kept in a constant pitch position, namely the “below rated pitch position”. Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions however depends on the complete design of the wind turbine. And in the supra-nominal zone, the pitch angle of the blades is changed in reaction to a change in rotor speed.
If a sudden wind gust occurs, i.e. a significant increase in wind speed in a relatively short time (extreme conditions), due to the inertia of the rotor, the rotor speed will not immediately increase. As a consequence, also the pitch system will not immediately react to the increase in wind speed. With the same pitch angle, stall may occur in the wind turbine blades, since the angle of attack of the blades may be above a critical angle of attack (because the wind speed changes and the rotor speed cannot track said change because of its own inertia).
Thus, in order to maintain an optimum angle of attack at the moment of the sudden wind gust, the pitch angle should be increased. Nevertheless, as the pitch system depends on the rotor inertia, it cannot track a sudden wind change, so blade pitch remains somewhat stuck, thus resulting in a large angle of attack. Depending on the precise effects of the wind, and the inertia of the rotor, it may be that the rotor speed even decreases a little bit, due to the separation of the flow from the blades as stall occurs. In response to this decrease in rotor speed, the pitch system will reduce the pitch angle more, thus aggravating the situation by further increasing the angle of attack.
The above situation (sudden wind gust) may be particularly troublesome in case of e.g. a Mexican hat wind gust. Mexican hat wind gusts are defined in the IEC 61400-1 2nd edition 1999-02 standard, since they may be particularly dangerous wind gusts. This standard defines Mexican hat wind gusts at various speeds, and at various azimuth angles.
The loads a wind turbine suffers during such a wind gust are severe and may define design loads for the wind turbine. This is due to the decrease in wind speed, before the high increase in wind speed. When the wind speed decreases, the pitch system tries to adapt the blades to this decrease (the blades are initially rotated in such a way to increase the aerodynamic torque). With the pitch adaptation still on going, a significant increase in wind speed occurs. The aerodynamic torque and the thrust force on the hub can thus be very high. The pitch of the wind turbine will then start to be adapted to these new wind conditions. However, the wind speed keeps increasing and due to the inertia of the system, the pitch can possibly not be adapted quickly enough, thus leading to the wind turbine potentially stalling and suffering increased loads. A typical pitch system may have an inherent pitch limitation of approximately 5°/second. Such a pitch rate may in principle be fast enough to respond to wind variations occurring during operation of the wind turbine. In general, the limiting factor in operation of the pitch system may not be the pitch drive system but the means used to sense wind speed, i.e. rotor speed.
The international patent application WO2010060772 (A2) discloses a method for controlling and regulating an operational parameter of a wind turbine blade such as e.g. a blade pitch angle, a position of a flap, or other means for changing the aerodynamic surface of a blade. These are controlled on a wind turbine during operation with the purpose of reducing any extreme tower loads. Based on a measured acceleration of the nacelle, the velocity of the wind turbine nacelle and a position of the wind turbine nacelle relative to a running mean are determined and from these the actual operating situation. This actual operating situation is compared to a predetermined space of acceptable operating situations determined from a set of normal operating situations. A control strategy is then chosen from a predetermined set of strategies. The method further comprises the steps of defining a control function for the operational parameter based on the chosen predefined control strategy, and controlling at least one of the operational parameters of at least one of the wind turbine blades in accordance with the control function.
The method of WO2010060772 (A2) may perform transitions from one strategy to another requiring a sudden reaction of the wind turbine, which may cause the wind turbine to operate in a too stressed manner. This may cause e.g. a significant mechanical wear of the wind turbine. Besides, definition of transition thresholds and implementation of optimum methods to perform said transitions can also be quite troublesome. Furthermore, in a wind turbine configured to work at a given pitch rate, the required reaction may be constrained by said given pitch rate, such that the expected reaction may not be fully/optimally performed.
There still exists a need for a method of operating a wind turbine that at least partially reduces the aforementioned problems. It is an object of the present invention to fulfil such a need.